The present invention relates to wavelength-selective devices usable in optical communication systems.
Optical fiber communication systems utilize wavelength-selective devices for various purposes as, for example, for routing light beams of different wavelengths to different destinations or as optical filters which allow light in a desired band of wavelengths to pass along the communication channel while removing or attenuating light at wavelengths outside of the desired band.
Wavelength-selective devices must meet demanding requirements for use in practical communications systems. The devices should be capable of separating wavelengths differing from one another by only a few nanometers. The wavelength-selective device should be environmentally stable, reliable and durable. Also, the wavelength-selective device should operate with a relatively low loss of optical power, i.e., the device should not dissipate substantial amounts of the optical power supplied to it in the desired wavelength bands.
Mach-Zehnder interferometers have been utilized as wavelength-selective devices in optical communication systems. As depicted in FIG. 1, a conventional Mach-Zehnder interferometer includes a pair of fibers F1 and F2. The fibers are coupled to one another at a first coupler C1 and a second coupler C2. The couplers are arranged to transfer light from one fiber to the other. As further explained below, the couplers may be so-called overclad tapered couplers in which narrowed, elongated portions of the fibers are closely juxtaposed with one another within a matrix or outer cladding. The couplers may be 3 dB couplers, arranged to transfer approximately one-half of the optical power supplied on one fiber to the other fiber. Fibers F1 and F2 have phase shift regions with different optical path lengths disposed between the couplers. Thus, the optical path length over the phase shift region in fiber F1 is different from the optical path length over the phase shift region in fiber F2. As used in this disclosure, the term "optical path length" is a measure of the time required for light at a given wavelength and in a given propagation mode to pass through the fiber from one end to the other. The optical path length difference has been provided by making the phase shift region of one fiber physically longer than the other, by making the two fibers F1 and F2 with different propagation constants so that the phase velocity of light within the two fibers is different, or both. The fibers can be provided with different propagation constants by making the fibers with different refractive index profiles. Where the fibers are "step-index" fibers, incorporating a core having a relatively high refractive index and a cladding with a relatively low refractive index overlying the core, the two fibers may have cores of different refractive indices, different core diameters, different cladding refractive indices or some combination of these. Regardless of the particular mechanism used to produce the optical path length difference, the single stage Mach-Zehnder filter depicted in FIG. 1 will direct light supplied through input 1 either to output 3 or to output 4 depending upon the wavelength of the light.
A typical single stage Mach-Zehnder filter has a substantially periodic transfer function relating the proportion of light directed to a particular output port to the wavelength of the light. That is, the amount of light appearing at any particular output port varies repetitively as the wavelength of the light varies. A typical transfer function for a single stage Mach-Zehnder device is illustrated in FIG. 2. It includes a series of alternating pass bands 5 and notches 6. At wavelengths within the pass bands, a substantial portion of the light supplied through port 1 is present at port 3; at wavelengths in notches 6, little or none of the light supplied through port 1 reaches port 3. The transfer function is periodic in that the pass bands and notches recur at substantially regular intervals along the wavelength axis. Although various characteristics can be achieved by coupling plural Mach-Zehnder devices in series, or by making each device with more than two optical path lengths, further improvements would be desirable.
In particular, there are needs for optical filters which will pass substantially all of the light within a single, relatively broad band of wavelengths, commonly referred to as a "pass band" and which will sharply attenuate light wavelengths lying just outside of the pass band. This need arises particularly in connection with optical amplifiers. An optical amplifier is a device which adds power to an optical signal. It is used principally to compensate for power lost in transmission through longer optical fibers. One form of an optical amplifier is known as an erbium-doped fiber amplifier (EDFA). The EDFA includes a length of fiber optic formed from special glass materials containing the element erbium. The input optical signal light beam, at a wavelength used for signal transmission is passed into the fiber along with light at another, shorter wavelength referred to as "pumping" light. Energy from the pumping light is absorbed and stored in the fiber. As the signal light beam passes through the fiber, this energy is released and incorporated into the signal light beam. Erbium-doped fiber amplifiers can be used with wavelengths in an operating band centered at about 1.55 micrometers. Ordinarily, the useful operating band of the amplifier is about 30 nm (0.03 micrometer) wide or more. Thus, the useful operating band of the amplifier may encompass wavelengths from about 1.53 micrometers to about 1.56 micrometers. This operating band is broad enough to permit simultaneous amplification of several different light beams at slightly different wavelengths.
Unfortunately, the EDFA also provides some amplification to light at wavelengths slightly outside of its useful operating band. Stated another way, the EDFA gain curve does not have a sharp cutoff at the edges of the operating band. Thus, where the incoming signal incorporates spurious components or "noise" at wavelengths slightly outside of the useful operating band, these spurious components will be amplified to some degree as well. Moreover, the amplifier itself can introduce noise at wavelengths that are slightly outside of the operating band. In both cases, the amplified noise passes downstream in the system and degrades system performance. Moreover, the optical energy taken from the fiber to amplify the noise is not available to amplify the desired signal. Thus, there is a substantial need for a simple filter which can be applied at the input or output of an EDFA to suppress signals lying slightly outside of the desired operating band of the EDFA, but which will pass substantially all of the wavelengths within the desired operating band without substantially attenuating them. In particular, there is a need for filters which can pass wavelengths from about 1.549 to about 1.565 micrometers while also suppressing signals with wavelengths from about 1.525 to about 1.545 micrometers. There are corresponding needs for optical filters with broad pass hands and sharp attenuation of wavelengths slightly outside of a desired pass band for use with other types of optical amplifiers and for use with other devices as well. There are also needs for the inverse type of filter, i.e., a filter which will suppress light at wavelengths within a broad band, but which will provide essentially unattenuated passage of light lying slightly outside of such band.